Late period

I was due for my period on Friday I haven't gotten and it's been four days now I'm not stressed or anything like that and I know that can be a factor but I really havent... I've been really tired lately sleeping all day and not being able to get up unless I have to use the bathroom that's how tired I am, I've been having nausea more in the mornings and at night I've been really moody and I was craving pickles the other day when I don't even like pickles I took a pregnancy test but it was negative can someone plz tell me what it might be and why I haven't gotten my period yet when I've been regular for years